This invention relates to the field of nondestructive testing, and especially the technique of electronic shearography. The invention comprises an improvement in the electronic circuitry used in shearography.
In the technique of shearing interferometry, or "shearography", two laterally-displaced images of the same object are made to interfere to form an interference pattern called a shearogram. The term "shearing" is used because of the lateral displacement of the interfering images. A first shearogram is taken while the object is in an unstressed condition, and another shearogram is taken when the object is stressed. Comparison of the two shearograms reveals information about the strain concentrations (and hence the integrity) of the object.
In the technique called "electronic shearography", the shearograms are stored in a computer memory, and are compared electronically to produce a composite pattern. Because all the data are processed electronically, the results of the analysis can be viewed in "real time".
U.S. Pat. No. 4,887,899 describes an apparatus and method for performing electronic shearography. In the apparatus shown in the cited patent, a shearogram is produced by passing light, reflected from the test object, through a birefringent material and a polarizer. The birefringent material, which can be a calcite crystal, splits a light ray, reflected from a point on the object, into two rays, and the polarizer makes it possible for these rays to interfere with each other. Thus, each point on the object generates two rays, and the result is a shearogram, i.e. an interference pattern formed by the optical interference of two laterally-displaced images of the same object.
It turns out that the spatial frequency of the shearogram produced with this arrangement is relatively low, because the effective angles between the interfering rays are small. Thus, the shearograms can be recorded by a video camera, which normally has much less resolving capability than a high-density photographic film. By storing the shearogram of the object in its initial, unstressed condition, and by comparing that shearogram, virtually instantaneously, by computer, with further shearograms taken under varying levels of stress, a "real time" image of the resultant strains on the object can be observed.
The above-described method can be practiced by storing the shearograms in separate frame buffers, and by using a real-time video subtractor to perform the comparison. Other methods of comparing the stored shearograms can also be used, as described in the above-cited patent. The amplified output of the subtractor is what is observed on a video display.
Each point on the actual shearogram is generated by the interference of light emanating from a pair of distinct points on the object. Thus, each pixel of the video camera is illuminated by light reflected from those two points. If the overall illumination remains constant, then any variations in the pixel intensity, in the shearogram, will be due only to changes in the phase relationship of the two points of light.
When the initial video image is stored, an initial intensity for each pixel is recorded, as described above. If any differential deformations occur in the object, such deformations will cause changes in the subsequent shearogram. In particular, the intensity of a given pixel will have changed according to the change in the phase relationship between the two rays of light (reflected from two points on the object) which illuminate the pixel. These phase differences can be either positive changes, causing the pixel to become brighter, or negative changes, causing the pixel to become darker. Whether the pixel becomes brighter or darker depends on the initial phase relationship and the direction of the change of phase. Due to the cyclic nature of phase interference, as the deformation of the object continually increases, the intensity at a given pixel will pass through a complete cycle. That is, the intensity at the pixel might increase to a maximum (positive) difference, then return to the original intensity, and then continue to a maximum (negative) difference, and so on.
In the systems of the prior art, only the positive-going variations appearing at the output of the subtractor (or other means of comparison) have been amplified. Thus, in the prior art, the negative-going changes are lost. That is, essentially half of the collected data are lost. The present invention provides an apparatus and method which avoids this waste of data, and thereby greatly improves the resolution of images obtained from electronic shearography.